Piston rings are mounted on pistons in engines on automobiles, for example. A piston ring slides with the wall face of a sleeve (cylinder) serving as a mating member. Hence, the piston ring is required to have low friction properties, high abrasion resistance and low aggressiveness to the mating member.
Conventionally, the sleeve serving as a mating member has been frequently made of cast iron, and the surface of the piston ring has been surface-treated; for example, a CrN (chromium nitride) film or a hydrogen-free hard carbon film (Diamond Like Carbon film: DLC film) is formed on the surface of the piston ring to provide the piston ring with the above-mentioned low friction properties, high abrasion resistance and low aggressiveness to a mating member.
However, in recent years, requests for improving the efficiency of engines and reducing the weights of engines from users have become strong. Hence, it has been examined that, instead of the cast iron having been used conventionally, aluminum alloy in which 15 atm % or more of Si is contained and to which Fe and Mn, for example, are added to enhance strength is adopted as the material of the sleeve.
However, as the result of the examination on the sleeve made of the aluminum alloy described above, it has been found that, in the case of piston rings having surfaces described below, the following problems will occur which correspond to the surfaces described below respectively.
First, in the case of a nitrided piston ring, there is a problem that the piston ring has insufficient adhesion suppression effect although the hardness of the piston ring increases. Furthermore, in the case of a piston ring having a CrN film formed on its surface, there is a problem that the piston ring is liable to cause adhesion to aluminum.
Further, in the case of a piston ring having a hydrogen-free DLC film formed on its surface, there is a problem that the piston ring exhibits high aggressiveness to a mating member and the abrasion amount of the sleeve made of aluminum alloy becomes very large although the piston ring exhibits high abrasion resistance because of its high hardness.
Still further, in the case of a piston ring having a hydrogen-containing DLC film formed on its surface, there is a problem that the abrasion amount of the hydrogen-containing DLC film becomes large because the film has low hardness although the abrasion amount of the sleeve made of aluminum alloy is very small because the hydrogen-containing DLC film has a low friction coefficient and low aggressiveness to a mating member. In particular, in the case that the above-mentioned sleeve made of aluminum alloy in which Si is contained to increase strength is used as a mating member, the abrasion amount of the hydrogen-containing DLC film becomes very large.
Under these circumstances, it has been proposed that an intermediate layer and a hydrogen-free DLC film are provided on the surface of a piston ring sliding with a sleeve made of aluminum alloy to improve abrasion resistance and scuffing resistance (for example, refer to Patent Document 1).